linux-kernel - Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

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Date:	Fri, 09 May 2014 21:19:32 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
>> If unfair lock is supported, the lock acquisition loop at the end of
>> the queue_spin_lock_slowpath() function may need to detect the fact
>> the lock can be stolen. Code are added for the stolen lock detection.
>>
>> A new qhead macro is also defined as a shorthand for mcs.locked.
> NAK, unfair should be a pure test-and-set lock.

I have performance data showing that a simple test-and-set lock does not 
scale well. That is the primary reason of ditching the test-and-set lock 
and use a more complicated scheme which scales better. Also, it will be 
hard to make the unfair test-and-set lock code to coexist nicely with PV 
spinlock code.

>>   /**
>>    * get_qlock - Set the lock bit and own the lock
>> - * @lock: Pointer to queue spinlock structure
>> + * @lock : Pointer to queue spinlock structure
>> + * Return: 1 if lock acquired, 0 otherwise
>>    *
>>    * This routine should only be called when the caller is the only one
>>    * entitled to acquire the lock.
>>    */
>> -static __always_inline void get_qlock(struct qspinlock *lock)
>> +static __always_inline int get_qlock(struct qspinlock *lock)
>>   {
>>   	struct __qspinlock *l = (void *)lock;
>>
>>   	barrier();
>>   	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>>   	barrier();
>> +	return 1;
>>   }
> and here you make a horribly named function more horrible;
> try_set_locked() is that its now.

Will do.

-Longman
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